Gavinfoxx said:

Psychic Blanks can't, no, because they're anathema to chaos and incapable of becoming possessed, but they can in theory be corrupted although by more mundane means.

Tau, orks and many other xeno species can't due to their biological makeup.

Chaos orks… no difference to normal orks.

Ghaundan said:

As far as I'm aware, the background has always held that no Grey Knight has ever fallen to Chaos. Also, I don't think a couple thousand people is a large group. The point is, this small group is  culled from a much, much larger group. Out of millions of potential candidates, those individuals who are proven "incorruptible" are formed into a group.

In fact, I'd say, if anything, the idea is somewhat diluted by the latest codex, since the Purifiers seem to indicate varying degrees of incorruptibility….

Ghaundan said:

I think there were some Chaos Orks in the earlier editions (or even during Rogue Trader?).

 yep. i think it was 2nd edition and they were khornate stormboys.

 Pre-Matt Ward, they were incorruptible. This isn't due to some strong will. This is due to the actual starter initiate (who by the by, must be a mortal of such unparalleled faith that they would undoubtedly have risen high within the Ecclesiarchy or Inquisition even had they been passed over) being more or less completely overridden with hypnoindoctrination. So the corruptibility is minimized by the fact that the idea of corruption itself is no longer compatible with the way they think. After that, they dedicate the entirety of their lives not burning the foes of Man to devotional prayers and cementing the ironclad faith in the Emperor. Furthermore, they have their entire skeletal frame etched in hexagrammic wards, and IIRC much of their skin as well. It's not that these are incorruptible men. These are men that have had the corruptibility removed from the hardware and software of their being at every turn, and then been sealed off to ensure that nothing corruptible might ever again cast a shadow on them.

Some people don't like that they were incorruptible, but I always did. Chaos is an immutable, powerful fact of existence, that always seems to be at the cusp of breaking humanity. I enjoy the fact that in one, small way, humanity has forged one, and keep this in mind as one, small bastion that even if destroyed, will never fall. 

Untouchables aren't immune to corruption. They are immune to Psychic powers and are incredibly damaging to daemons. So they could never stand to gain power from a Dark God or mutate from the powers of the Warp, but they could still fall to the temptation of Chaos, though it would be for less ethereal rewards then strength and speed and psychic might. Likewise Xenos are just born to have no soul, I would imagine. No soul, no chance of twisting to Chaos. Interpretation, this is.

Yeah. My feeling is if you are willing to bathe in the blood of the innocent you are already corrupted. Hexagrammatic wards on your pelvis or not.

Maybe the "cool factor" Matt Ward was going for backfired, I can't say, but I'll forever think of Grey Knights as Khorne's favored sons, not the favored sons of the Emperor after their Codex.

 Just pointing out that I don't count Matt Ward as canon. I know there is much debate about canon, but my Grey Knights don't bathe in Sororitas blood. That's called Khorne worship.

Seeten said:

Where there's a will, there's a way

And the gods of chaos have a whole damn lot of will

BangBangTequila said:

Who's Matt Ward? 

 you really do not want to know.

but seriously he works for games workshop and is renowned for taking sh@ts on everything that is cool about 40k and has a real fanboy obsession abut the ultramarines.

Grey Knights are inccoruptible. Period.
Until the fluff changes drastically which happens a lot.
But this topic crops up a lot on fora… and I find it… unusual.
Having a GK turn out corrupted would kinda break the point. They are the desitned final bulwark, the paladins in extremis. The best of the best. And the one group that against all odds has a staring contest with Chaos and doesn't blink.
They might loose but they won't 'turn'.
And I like it that way.

Alpha Legion marines were a welcome surprise. The decision to lump them in as unwitting Tzeentchites was….interesting. Also, inferno bolts are insane, as are Rubric Marines by extension. The book allows 1K Son Sorcerers to take them as regular minions; something I would probably change in my game.

On a side note, it will be interesting to see if any of the new powers will appear in the upcoming CSM codex. Daemonhunter indicated that FFG appears to have at least some advance notice of GW products.

Kiton said:

This. Fucking this.